The present invention relates generally to fiberoptic communications networks and, more specifically, to a method and an apparatus for controlling transmissions over fiberoptic networks.
While signals within telecommunications and data communications networks have traditionally been exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulated laser-generated light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals are often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, alternate optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. An isolated first switching element 110 is shown in FIG. 1. The optical switch of FIG. 1 is formed on a substrate. The substrate may be silicon, but other materials may be used. The silicon substrate includes planar waveguides defined by a lower cladding layer 114, a core 116, and an upper cladding layer (not shown). The core material is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers should be formed of a material having a refractive index that is sub-stantially different from the refractive index of the core material, so that optical signals are guided along the waveguides.
The core 116 is patterned to form an input waveguide 120 and an output waveguide 126 of a first optical path and to define a second input waveguide 124 and a second output waveguide 122 of a second optical path. The upper cladding layer is then deposited over the patterned core material. A chamber 128 is formed by etching a trench through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the location 130 aligned with the waveguides is filled with vapor or gas. Thus, TIR diverts light (A) from the input waveguide 120 to the output waveguide 122 to exit as light (B), forming route (Axe2x86x92B). When an index-matching fluid resides within the location 130 between the aligned waveguides 120 and 126, the light (A) propagates through the trench 128 to exit the switching element as light (D), forming route (Axe2x86x92D). The trench 128 is positioned with respect to the four waveguides such that one sidewall of the trench passes through the intersection of the axes of the waveguides.
The above-identified patent to Fouquet et al. describes a number of alternative approaches to alternating the first switching element between a transmissive state and a reflective state. One approach is illustrated in FIG. 1. The first switching element 110 includes two microheaters 150 and 152 that control the position of a bubble within the fluid-containing chamber 128. The fluid within the chamber has a refractive index that is close to the refractive index of the core material 116 of the four waveguides 120-126. Fluid fill-holes 154 and 156 may be used to provide a steady supply of fluid, but this is not critical. In the operation of the first switching element, one of the heaters 150 and 152 is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the location 130 of the intersection of the four waveguides. Consequently, an input signal along the waveguide 120 will encounter a refractive index mismatch upon reaching the chamber 128. This places the first switching element in a reflecting state, causing the optical signal along the waveguide 120 to be redirected to the output waveguide 122. However, even in the reflecting state, the second input waveguide 124 is not in communication with the output waveguide 126.
If the heater 150 at location 130 is deactivated and the second heater 152 is activated, the bubble will be attracted to the off-axis heater 152. This allows index-matching fluid to fill the location 130 at the intersection of the waveguides 120-126. The first switching element 110 is then in a transmissive state, since the input waveguide 120 is optically coupled to the collinear waveguide 126.
A concern with optical switching elements of the type shown in FIG. 1 is that in the transmissive state, a small amount of light is reflected from the switch. Reflection during the transmissive state is a result of the less than precise match between the indices of refraction of the fluid filling the chamber 128 and the waveguide core material 116. A precise match between the indices is difficult to accomplish because multiple considerations impact the selection of a fluid. For example, because the fluid is manipulated using thermal energy, the thermal properties of the fluid must also be considered. As the mismatch between the refractive index of the fluid and the refractive index of the core material increases, the portion of an optical signal that is reflected at the switch increases. Currently, the incidentally reflected light is not beneficially utilized in switching systems. On the contrary, if the incidentally reflected light leaks into an adjacent waveguide, the result can be undesirable crosstalk.
What is needed is an apparatus and a method for advantageously utilizing light that is incidentally reflected from a fluid-manipulable optical switch when the switch is in a transmissive state. What is further needed is such an apparatus and method that introduce little or no crosstalk.
An apparatus and a method for controlling transmissions over an optical network include at least one traffic detector connected to an optical switch to receive incidentally reflected light when the switch is in a transmissive state. By utilizing the detection of incidentally reflected light, the traffic detector non-intrusively monitors the conditions of signal transmission capabilities to the switch. Depending upon detection of a fault condition, the optical switch is set in a reflective state or a transmissive state.
The apparatus includes a fluid-manipulable chamber, two inputs and two outputs. The inputs and outputs are waveguides that channel optical signals to and from the fluid-manipulable chamber. Under fault-free conditions in which signal transmissions are detected to be normal, a first input is connected to a first output, while a second input is connected to a second output. However, when a fault condition is detected, the two inputs exchange outputs by reversing the fluid-manipulable chamber with respect to its ability to propagate optical signals through the chamber.
In a preferred embodiment, the fault-free condition is one in which the fluid-manipulable chamber is in the reflective state. In this state, there is an absence of fluid at the interfaces of the inputs with the walls of the chamber. The resulting mismatch of indices of refraction at the interfaces causes optical signals to be reflected. In this preferred embodiment, the reflected signals from the first input are channeled to the first output, and the reflected signals from the second input are channeled to the second output. At least one of the two outputs is coupled to a traffic detector that monitors traffic along the output. Preferably, each output includes a traffic detector. If one of the traffic detectors recognizes an unexpected absence of signal transmissions, the absence is interpreted as an indication of a fault along the corresponding input. In conventional SONET ring applications, the medium (e.g., a first fiberoptic line) that provides the input for one of the transmission paths is laced or otherwise physically coupled to the medium (e.g., a second fiberoptic line) that forms the output for the other transmission path. Consequently, if the input of the first transmission path is not propagating signals, it is presumed that the output of the second transmission path is also faulty. Likewise, a fault along the second input is likely to coincide with a fault along the first output. For this reason, the fault condition triggers the transmissive state of the fluid-manipulable chamber, thereby coupling the non-faulty input to the non-faulty output.
In the transmissive state, the fluid-manipulable chamber is filled with a fluid that has a refractive index that closely matches the refractive index of the light-carrying core material of the inputs and outputs. However, since there will be some mismatch in the indices of refraction, a small amount of each optical signal that is transmitted through the chamber will be reflected at the walls of the chamber. This xe2x80x9cincidentally reflected lightxe2x80x9d is used by the traffic detector or detectors to non-intrusively monitor the condition of the input which was determined to be faulty. When the appropriate traffic detector determines that signals are again flowing through the previously faulty input, the chamber is returned to its normal reflective state.
In an alternative embodiment, the transmissive state is the normal condition. The first input is aligned with the first output on opposite sides of the chamber. Similarly, the second input is optically aligned with the second output, so that the two are optically coupled when the fluid-manipulable chamber contains index matching fluid. In this embodiment, the traffic detector or detectors operate in the normal condition to monitor leakage light that is incidentally reflected as a result of a mismatch in the indices of refraction of the fluid and the core material through which optical signals are guided. An unexplained absence of incidentally reflected light is interpreted as an indication of a fault condition at a location upstream from the switch. It is also assumed that the output that is laced or otherwise physically coupled to the faulty input is malfunctioning. In order to minimize signal loss, the fluid-manipulable chamber is switched from the transmissive state to the reflective state, thereby coupling the non-faulty input to the non-faulty output.
While the fluid-manipulable chamber is in the reflective state, traffic detectors are used to determine when traffic is again being propagated along the input that was determined to be faulty. The transmissive state is re-established in response to determining that both inputs are functioning properly.
An advantage of the invention is that incidentally reflected light is utilized to monitor traffic within an optical network, such as a fiberoptic network. Thus, the monitoring occurs non-intrusively. Another advantage is that the detection of a fault condition automatically reroutes traffic, so as to avoid further signal loss.